Methanol reforming catalyst having a reduced volume shrinkage

ABSTRACT

A methanol reforming catalyst containing passivated copper and zinc oxide and/or alumina can be prepared by (1) precipitating or spray-drying a mixture of catalyst precursor components dissolved or suspended in a diluent in order to form a solid catalyst precursor in the form of powder or granules, (2) calcining and reducing the solid catalyst precursor obtained in stage (1), (3) passivating the reduced catalyst precursor obtained in stage (2) and (4) shaping the passivated catalyst precursor obtained in stage (3) to form the catalyst. A reduction in the volume shrinkage and an increase in the mechanical hardness during operation of the methanol reforming catalyst are achieved by the preparation process.

The present invention relates to a methanol reforming catalyst, aprocess for its preparation, its use and a process for steam-reformingmethanol.

In vehicles having a fuel cell drive, the hydrogen required isadvantageously produced from a liquid fuel during the drive itself. Inthe case of methanol as a fuel, this is effected primarily by means ofsteam reforming, autothermal reforming or partial oxidation. With theuse of hydrocarbons as fuel, it is possible to obtain hydrogen bypartial oxidation or autothermal partial oxidation. The CO alwaysconcomitantly formed as a main product or byproduct must either beremoved by a water gas shift or be oxidized by selective oxidation togive CO₂ which is not harmful to the fuel cell.

The steam reforming of methanol and the CO shift reaction areaccelerated by Cu-containing catalysts. These are in general substanceshaving the chemical composition CuO/ZnO/MeO where MeO is, for example,Al₂O₃, ZrO₂, La₂O₃ or Cr₂O₃. Such catalysts are prepared in oxide formand then generally activated in the reactor under reducing conditions,the CuO then being converted into elemental Cu, the actual catalyticallyactive species. The reduction of such catalysts is always associatedwith a volume and mass shrinkage of the catalyst molding. This istypically of the order of magnitude of from 10 to 25%. In a completelyfilled reactor (e.g. a tube-bundle reactor or a plate-type heatexchanger reactor), this leads to up to a fourth of the reaction spaceremaining unused. This is undesirable particularly in the case of mobilereformers which are designed to be as compact as possible.

The use of catalysts for steam production in mobile applications,primarily in cars operated using fuel cells, imposes general conditionswhich go well beyond those necessary in industrial applications.

Thus, owing to the small amount of space available in the car, the sizeof the reactors, too, is very limited. The reaction space present in thereactor must be completely filled with catalyst so that no excess emptyvolume is present. The empty space formed may eliminate the fixing ofthe catalyst. If the catalyst is present, for example, as a bed, thecatalyst pellets can then fly around during driving, owing to the highmechanical loads. This can lead to a substantial increase in theabrasion. This abrasion is undesirable since it can lead to blockages orother impairments of downstream components.

An additional problem may arise when the reformers are directly heated.Here, a heat transfer liquid is dispensed with and instead the heatrequired for the reforming is generated directly by catalytic combustionof hydrogen or methanol. In this method of heat generation, overheatingcan rapidly occur if parts of the reactor tubes or plates are not incontact with catalyst. Such overheating leads on the one hand tomaterial fatigue in the reactor but on the other hand can also lead tocoking of the fuel used.

The problem of volume shrinkage has long been known and proposedsolutions to it have also been described. For example, EP-A-0 884 272relates to the preaging of a Cu-containing catalyst by methanolreforming for about 50 hours. The preaging can be effected in a separatereactor or in the actual reformer reactor, it being necessary toreplenish the catalyst several times in the last-mentioned case.

EP-A-0 884 270 relates to another pretreatment process for the samecatalyst system. Here, the catalyst is preaged under an inert oroxidizing atmosphere at >300° C. However, the volume shrinkage in thistype of pretreatment is not comparable with the volume shrinkage whenthe catalyst is treated with hydrogen. The heating of a catalyst underoxidizing/inert atmospheres leads merely to decomposition of hydroxideor carbonate species still present from the original precipitatedproduct and, at higher temperatures, to burning-out of the tabletingaid, such as graphite or magnesium stearate. Such a treatment does notlead to reduction of the copper oxide component in the catalyst toelemental copper, but it is precisely the reduction of the copper oxideto copper that is primarily responsible for the volume shrinkage of thecatalysts since, in the reduction, the copper component, which ishomogeneously distributed in the carrier matrix on precipitation, isdissolved out of this and hence numerous microscopic cavities remain.This is all the more pronounced when it is considered that a typicalcatalyst precursor has a density of about 1 g/ml whereas metalliccopper, as present after the reduction, has a density of 9 g/ml.Although a catalyst heated under oxidizing/inert conditions suffers adecrease in volume, the volume shrinkage on subsequent activation underreducing conditions will be greater than the volume shrinkage duringheating without a reducing agent.

According to DE-A-198 01 373, the catalyst is reduced and thendeactivated again before introduction into the reactor.

Another constantly occurring adverse side effect in the reduction ofcatalysts is the substantial reduction in the mechanical stability.Particularly in the case of catalysts in tablet form, the hardness whenremoved (lateral compressive strength/end face compressive strength) isoften only a fraction of the initial hardness measured when the catalystwas still in oxide form. The low mechanical stability of the tablets ishowever undesirable in the case of mobile reformers. If, for example, acompacted bed of catalyst tablets is present, there is always a certainfriction of the tablets against one another during driving, which canlead to increased abrasion particularly at the corners and edges of thetablets. This abrasion is substantially independent of whether a cavityforms above the bed as a result of the volume shrinkage.

For obtaining a catalyst tablet which is mechanically stable in thereduced state too, there are various approaches which, on the one hand,aim at improving the active material and, on the other hand, alsodescribe tableting additives for increasing the mechanical stability.For example, DE-A-195 05 347 describes a process in which aCu-containing catalyst is tabletted by adding copper powder or aluminumpowder. This leads to a substantial increase in the hardness of thecatalyst in the reduced state too. However, a disadvantage of thisprocess is that the activity of such catalysts is always lower than theactivity of comparable catalysts without added metal.

It is an object of the present invention to provide a methanol reformingcatalyst and a process for its preparation, the catalyst beingdistinguished by low volume shrinkage and high mechanical hardness. Itis intended in particular to prepare copper-containing catalysts.

We have found that this object is achieved, according to the invention,by a methanol reforming catalyst containing passivated copper and zincoxide and/or alumina, which can be prepared by

-   -   (1) precipitating or spray-drying a mixture of catalyst        precursor components dissolved or suspended in a diluent in        order to form a solid catalyst precursor in the form of powder        or granules,    -   (2) calcining and reducing the solid catalyst precursor obtained        in stage (1),    -   (3) passivating the reduced catalyst precursor obtained in        stage (2) and    -   (4) shaping the passivated catalyst precursor obtained in        stage (3) to form the catalyst.

We have found that this object is furthermore achieved by a process forthe preparation of such a catalyst, said stages being carried out.

The expression passivated copper means at least surface oxidation of thecopper cluster in the catalyst to form at least a surface copper oxidelayer. Passivated copper is stable and non-pyrophoric in the air. Theterm passivated copper can also include completely oxidized copper andhence copper oxide. However, this expression is preferably understood asmeaning that the copper cluster or copper crystallites are oxidized onthe surface so that they are not pyrophoric in air.

The novel methanol reforming catalyst preferably contains passivatedcopper and zinc oxide and/or alumina as main components. It maysubstantially comprise these components, only small amounts of otheringredients being present. The catalyst may also consist of passivatedcopper and zinc oxide and/or zirconium oxide.

A catalyst prepared in this manner is preferably used, according to theinvention, for steam-reforming methanol, but can also be used forsteam-reforming higher alcohols, such as C₂₋₂₀-alkanols andhydrocarbons, for steam-reforming these hydrocarbons with addition ofair or as a CO shift catalyst.

In a process for steam-reforming methanol by reacting methanol and waterover such a catalyst, a pressure of from 0.5 to 10 bar and a temperaturefrom 150 to 450° C. are preferably employed.

A large number of methanol reforming catalysts can be prepared by thenovel process. Such catalyst compositions are described, for example, inDE-A-197 39 773 and EP-A-0 296 734.

Catalysts which contain (passivated) copper, zinc oxide and alumina areparticularly preferred. Some or all of the zinc oxide may be replaced byother divalent metal ions, such as Mg²⁺, Ni²⁺ or Mn²⁺. Some or all ofthe alumina may be replaced by other trivalent or tetravalent metaloxides, for example by zirconium, chromium, lanthanum, etc. In general,salts or oxides of one or more elements of the platinum metals, ofgroups 4, 5 and 11 and of the lanthanides of the Periodic Table of theElements can additionally be introduced in the preparation.

The present invention relates in particular to copper-containingcatalysts as used in fuel cell vehicles for methanol reforming and forthe CO shift reaction. Such catalysts are distinguished by very goodactivity, which simultaneously permits small amounts of catalysts. Thisis a necessary condition for use in cars. Such catalysts are typicallyreduced with hydrogen in the reactor before the beginning of thereaction. Copper oxide is reacted with hydrogen to give copper andwater. The metallic copper is present in the form of very small clusterswhich, for example, have a diameter of a few nm. They form the actualcatalytically active species. For the reduction of copper-containingcatalysts, such as methanol catalysts, synthesis catalysts or TTKcatalysts, there are precise and detailed specifications, cf. forexample Catalyst Handbook, Second Edition, Wolfe Publishing Ltd. 1989.

The novel catalysts have a combination of low volume shrinkage and highmechanical strength.

Both are achieved by reducing a catalyst precursor with hydrogen duringthe preparation itself and then passivating it again with oxygen. Thedried and calcined precipitated powder, which is generally present inthe form of spray-dried powder in industrial production, isadvantageously used for this purpose. The powder pretreated in thismanner can then be further processed to give moldings, possible stepsincluding the following:

-   -   Precompaction and tableting of the powder pretreated according        to the invention to give tablets.    -   Preparation of a slurry, kneading/mixing in a pan mill and        extrusion to give extrudates.    -   Preparation of a slurry, kneading/mixing in a pan mill and        extrusion to give complex moldings, e.g. monolithic structures        or catalyst sheets with or without a secondary structure.    -   Application of the catalytically active material to inert or        likewise catalytically active supports by means of Hicoating or        similar methods.

In all processes, the use of binders and additives is of course alsopermitted. There are also numerous other possibilities for furtherprocessing.

The procedure described permits the production of moldings having

-   -   low volume shrinkage during operation as a catalyst and    -   high mechanical stability during operation in the reduced state.

The reduction of a catalyst precursor with hydrogen has also been usedto increase the activity of the catalyst. EP-A-0 296 734 discloses thisprocess for increasing the copper surface area in Cu-containingcatalysts. As a result, the activity of the catalyst also increases, asdescribed for the water gas shift reaction in the abovementioned patent.According to EP-A-0 296 734, however, the reduction is carried out usingan intermediate which was not calcined beforehand. Furthermore, thetemperature during the reduction with hydrogen may not exceed 200° C.Catalysts prepared by this method are however mechanically not verystable since they primarily still comprise metal carbonate and basiccarbonate phases. Catalysts pretreated in this manner are very suitablefor steady-state applications but not very suitable for mobile use.

According to the invention, the catalyst is first calcined atpreferably >300° C. and then reduced or calcined at preferably <300° C.under reducing conditions. After the reduction, the catalyst ispassivated by oxygen or air at least until further handling of thecatalyst under air is possible. The unpassivated catalyst would bepyrophoric. A suitable reducing agent is primarily hydrogen, but it isalso possible alternatively to use any desired other reductions.

Inter alia, the following process variants are possible for thereduction of the catalyst with hydrogen:

-   -   isothermal procedure with constant hydrogen concentration    -   isothermal procedure with continuously increasing hydrogen        concentration    -   continuous increase in the temperature from room temperature        (minimum) to 500° C. (maximum) with constant hydrogen        concentration    -   continuous increase in the temperature from room temperature        (minimum) to 500° C. (maximum) with likewise continuously        increasing hydrogen concentration.

The reduction is preferably carried out, at least initially, usingdilute hydrogen, an inert gas, such as nitrogen or helium, beingpossible for dilution. Typical hydrogen concentrations are from 1 to 5%;however, it is also possible to employ pure hydrogen toward the end ofthe reduction. Throughout the process, it should be ensured that theexothermic nature of the reaction remains controllable. Furthermore, theagglomeration of the resulting copper crystallites as a result ofsintering is accelerated if the reduction is too rapid, leading to aconsiderable decline in the catalyst activity.

The subsequent passivation is preferably likewise carried out, at leastinitially, using dilute oxygen (dilute air or another oxidizing agent).Process sequences analogous or similar to those in the reduction arepossible.

If passivation is effected using exclusively dilute air at roomtemperature (e.g. 1% of air in nitrogen), the clusters are onlyexternally passivated, i.e. a covering of copper oxide forms. Elementalcopper is still present in the interior of the clusters. Such a state isvery much more likely to apply to the novel pretreatment than strongerpassivation at higher oxygen concentrations or temperatures since, underthese conditions, all the copper reduced beforehand is reoxidized.Accordingly, only external (surface) passivation is preferably effected.

A possible technical solution is, for example, the coupling of tworotating tubes, separation of the atmospheres being ensured by means ofan inert lock. Hydrogen or oxygen (air) can be passed countercurrentlyto the catalyst powder in the respective rotating tube, with the resultthat an advantageous concentration gradient is achieved.

The shaping in stage (4) preferably leads to layers, extrudates,monoliths, strands, pellets or tablets.

In stage (2), the calcination and reduction can be carried outsimultaneously (calcination in reducing atmosphere) or in succession, itbeing possible for the calcined catalyst precursor to be comminuted inbetween.

In addition to copper and zinc oxide, the catalyst may additionallycontain alumina and further oxides. Solutions of zinc, aluminum andcopper salts may be precipitated simultaneously or in any desiredsequence in stage (1).

Stages (1) and (2) are particularly preferably carried out as follows:

-   -   (a) precipitation of a solution of zinc and aluminum salts, the        Zn:Al atomic ratio being from 3:1 to 1:3, with an alkali metal        carbonate or hydroxide solution at a pH of from 5 to 12 and a        temperature of from 20 to 100° C.,    -   (b) isolation and washing of the precipitate to remove alkali        metal ions,    -   (c) drying of the precipitate,    -   (d) calcination of the precipitate at from 250 to 800° C. to        give a mixed oxide,    -   (e) dispersing of the mixed oxide in an acidic solution of        copper and zinc salts, the Cu:Zn atomic ratio in the solution        being from 1:5 to 20:1,    -   (f) precipitation of the dispersion with an alkali metal        carbonate or hydroxide solution at a pH of from 6 to 9 and a        temperature of from 20 to 100° C.,    -   (g) performance of steps (b) to (d),    -   (h) reduction of the catalyst precursor obtained in stage (g)        with a gas containing free hydrogen.

Particularly preferred catalysts have a Cu:Zn atomic ratio of from 1:5to 5:1, particularly preferably from 1:1 to 4:1, in particular from 2:1to 3:1. The (Cu+Zn):Al atomic ratio is preferably from 99:1 to 70:30,particularly preferably from 95:5 to 80:20. A Cu:Zn:Al ratio of about65:25:10 is especially preferred.

This corresponds to a composition of about 67% by weight of Cu, 26.4% byweight of ZnO and 6.6% by weight of Al₂O₃ in the prepared catalyst.

In addition to the three elements Cu, Zn and Al, further elements mayalso be introduced into the catalyst, such as platinum metals andelements of groups 4, 5 and 11 and of the lanthanides of the PeriodicTable of the Elements. Preferred examples are Pd, Pt, Rh, Ln, Os, Au,Zr, Ti, V, Nb, Ta and the lanthanides.

Preferred novel catalyst compositions are described in DE-A-197 39 773.

In stage (1), the drying is preferably carried out at from 20 to 400°C., particularly preferably from 50 to 200° C., in particular from 80 to130° C. The calcination in stage (2) is preferably effected at from 200to 800° C., particularly preferably from 250 to 600° C., in particularfrom 300 to 500° C.

The novel catalysts have a very low volume shrinkage in combination withonly a slight loss of lateral compressive strength during operation inthe reactor, in particular as a motor vehicle catalyst. The novelprocess steps are carried out during the catalyst preparation itself.There are no additional, expensive forming steps.

If the passivation of the catalyst is carried out under mild conditions,i.e. low temperatures (<50° C.) and low hydrogen partial pressures areemployed, it is possible for the copper cluster present after thereduction to be only surface-passivated with a layer of Cu₂O. The coreof the cluster furthermore consists of metallic copper. A catalystprepared in this manner can be handled under air and, after filling of amobile reformer, can be very easily reactivated there with only moderateevolution of hydrogen since a major part of the copper is still presentin metallic form. If, on the other hand, an unpretreated catalyst isintroduced into such a reformer, the catalyst must be completelyactivated in the reformer, which is associated with a strong exothermicreaction and very long times for the procedure. A catalyst pretreatedaccording to the invention can accordingly substantially simplify thestartup in the car by suitable reduction/passivation.

The present invention also relates to a process for reducing the volumeshrinkage and for increasing the mechanical hardness during operation ofmethanol reforming catalysts, in which the methanol reforming catalystis prepared by the process described above.

An example which follows illustrates the invention. In addition,corresponding counter-examples are intended to show that, when othermethods are used, greater complexity is required in order to achieve thesame target parameters with regard to volume shrinkage and hardness.When such methods are used, catalysts which have either hardness or lowvolume shrinkage are generally obtained. However, the two together arenot possible according to the prior art.

EXAMPLE 1

A copper catalyst based on copper carbonate is prepared analogously toEP-A-0 296 734 (Example 1). The composition in atom % is: Cu=65%,Zn=25%, Al=10%. The precipitated product of the second precipitationstage is washed sodium-free and is dried at 120° C. Thereafter, theproduct is calcined at 300° C. for 4 hours and is comminuted to givefrom 0.5 to 0.7 mm chips.

4060 g of these chips are installed in a heatable tubular furnace(diameter: 100 mm, height: 1000 mm). The furnace is heated at 200° C.and a mixture of 1% of H₂ and 99% of N₂ (total: 100 l(S.T.P.)/h) ispassed through the chips for 16 hours.

The hydrogen is then shut off and the furnace is cooled to roomtemperature. For passivation, the nitrogen is gradually enriched withair so that the temperature in the catalyst never exceeds 50° C. The endpart of the passivation is reached when all nitrogen has been exchangedfor air. The removed catalyst has a 14.2% lower mass and a 14% lowervolume than the installed catalyst.

The chips are then precompacted and tableted on a tablet press to givesmall 1.5×1.5 mm tablets. The tableting is carried out in such a waythat the lateral compressive strength of the tablets is about 40 N. TheBET surface area of the tablets is 51 m²/g.

COMPARATIVE EXAMPLE 1

A copper catalyst based on copper carbonate is prepared analogously toEP-A-0 296 734 (Example 1). The composition in atom % is: Cu=65%,Zn=25%, Al=10%. The precipitated product of the second precipitationstage is washed sodium-free and is dried at 120° C. Thereafter, theproduct is calcined at 300° C. for 4 hours and is comminuted to givefrom 0.5 to 0.7 mm chips. The chips are converted directly into 1.5 mmtablets, a lateral compressive strength of about 50 N being established.The BET surface area of these tablets is 65 m²/g.

Catalyst Test:

The catalysts are introduced into a tubular reactor (diameter 10 mm;amount introduced=10 ml) and a 1.5 m/m mixture of methanol and water ispassed over said catalysts at 280° C. and 2 bar. The catalysts areoperated under these conditions for from 30 to 40 hours. Both catalystshave a comparable methanol conversion and hence also a comparablehydrogen evolution. The volume shrinkage and lateral compressivestrength of the catalysts removed are measured. The results aresummarized in Table 1: TABLE 1 Lateral compressive strength and volumeshrinkage after catalyst test Catalyst from Catalyst from Example 1Comparative example 1 Catalyst before test Lateral compressive 39.5 51.0strength [N/tablet] Catalyst after test Lateral compressive 34.2 7.4strength [N/tablet] Volume shrinkage 12.5 22.4 [%]

COMPARATIVE EXAMPLE 2

The catalyst from Counter-example 1 is subjected to a repeated oxidationand reduction cycle before being installed in the test reactor. Duringthe reduction, the hydrogen content is gradually increased to 3% byvolume, and the oxygen content during the passivation is from 0.5 to1.0% by volume. The cycle is carried out 5 times. After the end of thepretreatment, the catalyst has shrunk by about 20% by volume; thelateral compressive strength has decreased to a tenth of the originalvalue. The abovementioned test is then carried out with this catalyst.The MeOH conversion achieved is about 10% below the value of thecatalyst from Example 1. The catalyst removed after the end of the testhas the properties summarized in Table 2: TABLE 2 Lateral compressivestrength and volume shrinkage after catalyst test Catalyst from Catalystfrom Example 1 Comparative example 2 Catalyst before test (but afterpretreatment) Lateral compressive 39.5 3.5 strength [N/tablet] Catalystafter test (but after pretreatment) Lateral compressive 34.2 2.4strength [N/tablet] Volume shrinkage 12.5 1.5 [%]

The results show that, as a result of the chosen pretreatment, thevolume shrinkage of the catalyst is reduced but at the same time thelateral compressive strength declines to such an extent that use in amobile reformer is no longer possible. The catalyst pellet fromComparative example 2 can be crumbled with the hand without problems.

1. A methanol reforming catalyst containing passivated copper and zincoxide and/or alumina, which can be prepared by (1) precipitating orspray-drying a mixture of catalyst precursor components dissolved orsuspended in a diluent in order to form a solid catalyst precursor inthe form of powder or granules, (2) calcining and reducing the solidcatalyst precursor obtained in stage (1), (3) passivating the reducedcatalyst precursor obtained in stage (2) and (4) shaping the passivatedcatalyst precursor obtained in stage (3) to form the catalyst.
 2. Acatalyst as claimed in claim 1, wherein the shaping in stage (4) leadsto layers, extrudates, monoliths, strands, pellets, tablets or chips. 3.A catalyst as claimed in claim 1, wherein the calcination and reductionare carried out in succession in stage (2), it being possible for thecalcined catalyst precursor to be comminuted in between.
 4. A catalystas claimed in claim 1, wherein the catalyst contains alumina in additionto passivated copper and zinc oxide, and solutions of zinc, aluminum andcopper salts are precipitated simultaneously or in any desired sequencein stage (1).
 5. A catalyst as claimed in claim 4, wherein stages (1)and (2) are carried out as follows: (a) precipitation of a solution ofzinc and aluminum salts, the Zn:Al atomic ratio being from 3:1 to 1:3,with an alkali metal carbonate or hydroxide solution at a pH of from 5to 12 and a temperature of from 20 to 100° C., (b) isolation and washingof the precipitate to remove alkali metal ions, (c) drying of theprecipitate, (d) calcination of the precipitate at from 250 to 800° C.to give a mixed oxide, (e) dispersing of the mixed oxide in an acidicsolution of copper and zinc salts, the Cu:Zn atomic ratio in thesolution being from 1:5 to 20:1, (f) precipitation of the dispersionwith an alkali metal carbonate or hydroxide solution at a pH of from 6to 9 and a temperature of from 20 to 100° C., (g) performance of steps(b) to (d), (h) reduction of the catalyst precursor obtained in stage(g) with a gas containing free hydrogen, it being possible for thesolutions in steps (a) and/or (e) additionally to contain salts oroxides of one or more elements of the platinum metals, of groups 4, 5and 11 and of the lanthanides of the Periodic Table of the Elements orfor the salts or oxides to be applied to the mixed oxides.
 6. A processfor the preparation of a catalyst as claimed in claim 1, wherein saidstages are carried out.
 7. A process for steam-reforming methanol byreacting methanol and water over a catalyst, as defined in claim 1, atfrom 0.5 to 10 bar and from 150 to 450° C.
 8. A process for reducing thevolume shrinkage and for increasing the mechanical hardness duringoperation of methanol reforming catalysts, wherein the methanolreforming catalyst is prepared by a process as claimed in claim 6.